Export custom mtl file
Ni (optical_density)

The code lives in scripts/addons/io_scene_obj/export.obj (line 92+ ). You could comment out these lines:

if hasattr(mat, "raytrace_transparency") and hasattr(mat.raytrace_transparency, "ior"):
    fw('Ni %.6f\n' % mat.raytrace_transparency.ior)  # Refraction index
else:
    fw('Ni %.6f\n' % 1.0)


The .obj exporter will automatically add the Ni as soon as you assign any material to the object.

Could be worth updating your C++ obj file loader, at least to ignore the Ni command if material is set, because it is a valid part of the .mtl specs